Corrosion electro-potentiometer probe



Jan. 19, 1965 G. R. LLOYD 3,155,485

CORROSION ELECTRO POTENTIOMETER PROBE Filed Jan. 31, 1963 3 Sheets-Sheet 1 Figure "I PROCESS STREAM i TESTCNG ZONE 2 Jan. 19, 1965 G. R. LLOYD 3,166,485

CORROSION ELECTRO-POTENTIOMETER PROBE Filed Jan. 31, 1963 s Sheets-Sheet 2 Z 9 5 l O (0 Z 2 :0 Lu If) Z L 8 O E 8 E 3 m j :1 I 2 o O 8888888888o888 N (O m '8' an) N N m '0 N snom'ruw GORDON R. LLOYD Inventor Patent Attorney Jan. 19, 1965 G. R. LLOYD 3,166,485

V CORROSION ELECTRO-FOTENTIOMETER PROBE Filed Jan. 31, 1963 3 Sheets-Sheet 3 FIGURE 4 WATER WASHING RATE Vs. POTENTIAL Corrosion Electro-Potentlometer Probe In Cyanide Containing Stream I I I No water wash POTENTIAL UNITS 3O bbls/ hr. water I 0 2 4 6 8 IO l2 l4 i6 I8 20 22 24 26 HOURS GORDON R. LLOYD inventor Patent Attorney United States Patent 3,166,485 CORROSION ELECTRD-PQTENTIOMETER lPROBE Gordon R. Lloyd, Montreal, Quebec, Canada, assignor to Esso Research and Engineering Company, a corporation of Delaware Filed Jan. 31, 1963, Ser. No. 255,236 1 Claim. (Cl. 204--1) The present invention relates to a method for accurate corrosion control. In particular, this invention involves the use of specially constructed probes containing copper and steel electrodes to continuously determine the cyanide concentration of a liquid stream by measuring the extent of potential reversal. v

It is well known in the art that copper is cathodic to steel in most aqueous solutions, for instance, such as in chloride solutions. However, when the aqueous solution contains appreciable concentrations of cyanide, the normal potentials are reversed and the copper then becomes anodic to steel. This results in an accelerated corrosion rate for any process equipment. fabricated from copper-steel alloy which contacts these cyanide solutions over a period of time.

Cyanides in petroleum process streams are the product of the hydrolysis of cyanogen, which in turn is formed by the reaction of nitrogenous compounds in the catalytic cracking reactions. Since nitrogen compounds are more prevalent in heavy crude fractions, cyanide induced corrosion is more severe in plants where heavy feed stocks, such as those prepared by deasphalting, are cracked,

Nitrogenous compounds which are found in crude oil include porphyrins, pyrroles, pyridine, piperidine, indole, quinoline and skatole. The cyanogen produced by the decomposition of the above compounds may be hydrolyzed to yield these corrosive compounds:

1 E20 HCN HOCN Cyanogen Hydrocyanic acid Cyanic acid (2) ON C 0 OH' N 0 OH Oxalic acid Furthermore, the cyanogen may react with the hydrogen sulfide found extensively in petroleum streams to yield other corrosive compounds as follows:

CN N H23 HON HSCN Thiocyanic The corrosive processinvo'lves the attack of the above type nitrogen compounds upon the copper-base alloys. The cyanide ion present reacts with the copper to form copper cyanides. These copper cyanides can then react further Fatented Jan. 19, 1965 ice in the following equation:

Cyanide cyanide complex The net result is a solubilizing of the copper present in the alloy with a simultaneous loss in the structural strength of the equipment fabricated from the alloy. However, it should be pointed out that the above equations are only examples of the various types of corrosion reactions and serve as a rather simplified explanation of corrosion mechanisms. The actual corrosion products are no doubt much more complex and may include such compounds as copper ferricyanide Cu (Fe(CN) and tetrammine copper sulfate (Cu(NH )SG -H O.

It is therefore quite obvious that it is of extreme importance to the oil processing industry to be able to determine the presence of corrosive quantities of nitrogen compounds, such as the cyanides, in the process streams so as to be able to quickly reduce their concentration by the injection of water into the stream. Water injection is herein meant to include water injection as such or Water treated with chemical additives, e.g., sodium polysulphide, to counteract the corrosivity of the cyanides.

It is possible to check the cyanide concentration of plant streams by running a series of control tests on aliquots taken periodically from the stream. This would involve the use of standard chemical titrations such as the one employing silver nitrate described in Elements of Quantitative Analysis, 4th Edition, by Willard et al., 1956, pages 133-135. Such a control system, however, has many basic defects which make it unattractive in an industrial process. In the first place, convenient access must be obtained to the process stream to allow withdrawal of samples but at the same time the sample port must contain sufiicient safety devices so as to prevent the escape of any toxic cyanide containing solution. This of course increases the expense of the basic equipment needed in the process. Then the sample must be transported to a laboratory where it would undergo a rather complicated titration procedure. Each handling step requires extensive safety precautions and results in a laborious, inefiicient and potentially dangerous control procedure. Furthermore, there would be a large time lag between the moment of sampling and the completion of the analysis, which lag would complicate the control of the water injection treatment for the removal of the contaminant cyanides.

The present invention involves the use of specially con- If an exact knowledge of the total concentration of cyanide is desired, it can be easily obtained by either running standard solutions past the electrodes or else by determining the cyanide concentration by titration for various potential values and then in both cases plotting a standardization curve of potential v. cyanide concentration. Further advantages to be derived from the present system include the possibility of automatically controlling the water injection system by having it respond to predetermined signals from the detector unit when the cyanide concentration rose to a value considered to be critical.

The nature and substance of this invention may be more clearly perceived and fully understood by referring to following description and claim taken in connection with the accompanying drawings in which:

FIG. 1 represents a block and line diagram of the corrosion control system installed in a process stream;

FIG. 2 represents a cross-sectional view of the test probe;

FIG. 3 is a graph showing the general relationship of the potential difference between a copper alloy electrode and a steel electrode and increased cyanide concentrations; and

FIG. 4 is a graph showing the sensitivity of the probes to changes in cyanide concentration due to difierent rates of water injection.

Referring now in detail to the drawings, FIG. 1 shows a process stream entering the testing zone 2 by means of line 1. The stream is initially contacted by a probe 3' containing a steel electrode protruding into the interior of the said testing zone. At a further distance downstream, the process stream is contacted by a second probe 3' which contains a copper alloy electrode 4 protruding into the interior of the above said testing zone. Both electrodes are connected by electrically conducting leadsto an indicating instrument 6 which may be a meter, recording device or similar type of equipment. The process stream leaves the testing zone by means of line 7 and returns to the refining system.

Referring now to FIG. 2, the construction of the corrosion probe is shown in cross-sectional detail. The probe 3 consists of an electrode element (made of steel 4 or copper alloy i) jacketed by a tight fitting Teflon bushing 8 which serves to provide a non-corroda'ole, leak proof seal between the electrode and the surface wall of main plug 10. Furthermore this construction is designed to provide for infiinity resistance between the electrode and the main plug. Tefion bushing 8 has a central concave bulge 9 which serves to intimately contact plug 10. Plug '12 fits onto the bushing so that the plugs sides rest upon the uppermost portion of the bushings bulge. This plug can be fitted with external threads 13 which fit into corresponding internal threads 14 on the inner edges of plug 1%. Meshing the above grooves results in securing the Teflon bushing into sealing position and serves to hold the probe within the main plug 10. The entire probe assembly 3 is secured to the testing zone by meshing external threads 15 on plug 10, with internal threads 16 on the testing zone wall 11. It is possible, of course, to secure plug 10 to wall 11 by other means, such as welding. If this were the case, access to the electrodes would still be easy by unscrewing plug 12 Therefore, maintenance of the electrodes can be accomplished quite easily but with no concomitant loss to the leakproofness nor corrosion resistivity of the system,

When using the above probe system under high pressure conditions, it is advisable, for safety reasons, to install a retaining clamp to hold the electrode. In using a clamp, it is necessary to insert an insulating gasket similar to bushing 8, between the electrode and the clamp plate to ensure an infinite resistance. 9

Referring now to FIG. 3, a graph indicating the relationship of the potential diiference between a steel electrode and a copper alloy electrode and increasing cyanide concentration is shown. This graph discloses that a copperiron couple can accurately respond to a change in the cyanide concentration of the solution in which it is introduced. Here the couple potential, in millivolts, undergoes reversal respective to the iron potential as cyanide, in the form of a 5% KCN solution, is added. Curves, such as depicted in this figure are useful in the standardization of the corrosion control equipment. 9

Referring now to FIG. 4, a graph indicating the relationship between the water injection rate and the millivolt potential of a process stream determined by a copper and a steel corrosion probe is shown. These results were obtained in an actual field test of corrosion control system, essentially of the type described in FIG. 1, in a light ends condenser stream. The graph indicates that this system can accurately indicate'the change in the cyanide concentration in a process stream which would thereby allow effective and efiicient removal of this corrosioninducing material.

What is claimed is:

A method for continuously determining the cyanide concentration in a process stream comprising contacting said cyanide-containing process stream with a steel electrode and a copper alloy electrode, said electrodes being connected by an external circuit, whereby a potential reversal proportional to the concentration of cyanide in the process stream is induced in said circuit, and measuring said induced potential reversal.

References Cited in the file of this patent UNITED STATES PATENTS Ange-ll Mar. 3, 1914 

